Effects of nifedipine, BAY K 8644 and ryanodine on phenylephrine-induced mechanical responses in papillary muscles.(A) The representative response to phenylephrine (10 μM) in C57BL/6J. Actions of nifedipine (0.3 μM, B), BAY K 8644 (1 μM, C) and ryanodine (0.3 μM, D) on twitch tension in the absence and presence of phenylephrine in C57BL/6J. Phenylephrine exerted positive inotropic response in the presence of ryanodine (i.e., in situations where SR-function is greatly reduced). Summary of the effects of BAY K 8644 (1 μM, E) and ryanodine (0.3 μM, F) on phenylephrine-induced response (n = 4–6, *P<0.05). (G) The levels of papillary muscle MLC2v phosphorylation in the presence and absence of BAY K 8644 or ryanodine in C57BL/6J and C57BL/6N. The % ratio of phosphorylated MLC2v vs. total MLC2v was calculated. BAY K 8644 and ryanodine did not modify MLC2v phosphorylation levels. The significance in the differences in the MLC2v phosphorylation levels was preserved in the presence of BAY K 8644 and ryanodine between C57BL/6J and C57BL/6N (n = 4–6, *P<0.05). Atenolol was present throughout the experiments for tension measurements.
{}